Crocker and Henderson invented a system of odor classification which has four basic odors which they termed the odor square:

Fragrant

Acid

Burnt

Caprylic

They marketed the Crocker-Henderson Odor Classification Set as an educational tool. It contained vials of odorants for comparison, each labled with the basic odor and its position on a nine point scale (odors are rated 0 - 8 ).